In the production of superalloys (nickel, cobalt and/or iron base) many benefits are obtained with the addition of a small content of boron and/or zirconium, for example, to reduce strain-aging cracking problems. It is also known in the art that these elements, at certain content levels, may be harmful in products in the welded form. U.K. Patent Application No. GB 2,183,675-A discloses among other features, the elimination of zirconium from the well-known WASPALOY alloy to improve weldability of cast products. U.S. Pat. No. 4,787,945 teaches the criticality and interdependence between zirconium and nitrogen contents in the composition of certain alloys. The two elements must be present "to effect grain size control" of the alloy.
An article entitled "A Study of the Weldability and Weld-Related Microstructure of CABOT alloy 214," by M. J. Cieslak, et al, in Metallurgical Transactions, Volume 19A, March 1988, discuses the harmful effects of boron and zirconium when welding a nickel-base alloy now known as HAYNES 214 alloy. HAYNES is a registered trademark of Haynes International, Inc., the assignee of the present invention.
U.S. Pat. No. 4,671,931 relates to a nickel-base alloy containing iron, chromium and aluminum. This patent constitutes an improvement over the alloys as defined in U.S. Pat. Nos. 4,460,542 and 4,312,682 known as NICRALY alloys generally containing boron and/or zirconium. These alloys, including HAYNES 214 alloy, may contain titanium as an optional element.
U.S. Pat. No. 1,746,209 relates to the use of a hydrogen-nitrogen mixture as a welding shield in electric arc welding. Ammonia is disclosed as a possible source of the H--N mixture. The use of mixed gases in this patent provides improved characteristics of the welding arc. Only a scant mention is made of a "nickel-chromium alloy" being welded. There is no mention of boron or zirconium in the patent.
The addition of nitrogen to metals and alloys has been well-known in the art for many years: for example, U.S. Pat. No. 2,069,204; U.S. Pat. No. 2,537,103; No. 4,588,450 and U.S. Pat. No. 4,619,692. These references relate to the melting and production of alloys or the addition of nitrogen to an alloy in solid state.
The use of nitrogen in welding Types 304 and 316 steel was disclosed in an article "Influence of Nitrogen Addition on Microstructure and Pitting Corrosion Resistance of Austenitic Weld Metal," by Kamachi Mudali, et al, in Werkstoffe un Korrosion (Materials and Corrosion), Vol. 37:12, pp 637-643 December 1986. The article was reported in Current Awareness Bulletin of the Materials and Ceramics Information Center at Battelle Columbus Division, Issue No. 167, June 1987. The article relates to the structure and pitting corrosion properties specifically of Types 304 and 316 stainless steels that normally do not contain boron or zirconium.
The prior art tends to suggest that there is a need for the beneficial effects of boron and zirconium but there is a problem with harmful effects of boron and zirconium in welded alloy products. The combined teachings of these disclosures do not suggest a solution to the problem of weld hot cracking of superalloys containing boron and/or zirconium.
It is therefore an object of this invention to provide a welding process that reduces, or eliminates, weld hot cracking in the welding of superalloys.
It is another object of this invention to provide such benefits through the use of a new shielding gas composition.
Other objects, features, and advantages of the present invention may be discerned by people skilled in the art from the following description of a presently preferred embodiment.